The recovery of cardiotomy fluid and venous blood during a cardiopulmonary bypass procedure involves collecting the fluid and blood in a reservoir and treating such fluid and blood for reinfusion into the patient. Treatment of the cardiotomy fluid and venous blood may include oxygenation, temperature control and circulatory pumping in a life sustaining extracorporeal blood flow. The cardiotomy fluid recovered from a surgical site additionally requires treatment to remove foreign material that is collected at the wound site by exposure to air and generated during the surgical procedure (i.e., tissue particles, blood clots or bone fragments, as well particulates from foreign body exposure, for example). Similarly, the venous blood received from the circulatory system is also treated to ensure that any entrained bubbles of gas created by extracorporeal circulation are removed. After treatment, the collected blood and cardiotomy fluid are returned to the patient via the extracorporeal circuit.
A variety of conventional separate or combined cardiotomy fluid and venous blood reservoirs have been developed to collect and treat cardiotomy fluid and venous blood during a surgical procedure. Typically, cardiotomy fluid collected within the reservoir is first subjected to a defoaming step by a defoamer element and then to a filtering step by a depth filter element. The cardiotomy fluid must be filtered and defoamed to remove air, debris and clots generated during the procedure. Since additional air bubbles may be created as the fluid flows through the filter element, the cardiotomy fluid often undergoes a subsequent defoaming step in a second defoamer element prior to reinfusion into the patient.
Venous blood collected in the reservoir must also be filtered and defoamed prior to being reinfused. However, there are two main differences between venous blood and cardiotomy fluid. First, unlike cardiotomy fluid, venous blood is relatively free of foreign material and clots. As a result, venous blood collected within the reservoir requires only minimal defoaming and filtration. Second, the flow rate of venous blood is, in general, significantly greater than the flow rate of cardiotomy fluid. Forcing venous blood together with cardiotomy fluid through the depth filter element used for cardiotomy fluid could damage healthy venous blood cells. Therefore, it is preferred to have specific filter and defoamer elements for cardiotomy fluid and separate filter and defoamer elements for venous blood. Alternatively, the same filter and defoamer elements can be used for both cardiotomy fluid and venous blood provided that the cardiotomy fluid and venous blood maintain separate flow paths through the filter and defoamer elements in the reservoir.
The formation of air and gas bubbles in conventional cardiotomy fluid and venous blood reservoirs is a common and on-going problem. Since the presence of air bubbles in the patient's vascular system may cause various life-threatening conditions, it is vital that all air or gas bubbles entrained in the venous blood and cardiotomy fluid are removed prior to reinfusing the fluid and blood into the patient. Conventional cardiotomy fluid and venous blood reservoirs incorporate various filters, defoamers, screens and similar devices in a variety of shapes and configurations to facilitate trapping and removal of bubbles from the fluid and blood.
An example of such a cardiotomy fluid and venous blood reservoir can be found in U.S. Pat. No. 4,737,139, issued Apr. 12, 1988, to Paul F. Zupkas et al. (the '139 patent). The '139 patent discloses a reservoir that includes upper and lower chambers which are vertically separated by a horizontal ring-like portion of a support member. The cardiotomy fluid which is received by an upper inlet is subjected first to a defoaming step by a defoamer and then to a filtering step by a depth filter element. Finally, the fluid is again defoamed by a defoamer element which is surrounded by a mesh. The venous blood is defoamed by a lower portion of the defoamer element which is shared with the cardiotomy fluid in parallel but fluidly separate flow paths. After flowing radially outwardly through the defoamer element, both cardiotomy fluid and venous blood flow into a common chamber for return to the patient via a lower outlet of the housing.
The cardiotomy fluid defoamer and filter elements and the venous blood defoamer and filter elements of the '139 patent have substantially the same diameter. Therefore, the effective defoamer areas in each of the venous blood and cardiotomy fluid flow paths are a function of the fractions of the height of the device which are devoted to each flow path. However, since the cardiotomy fluid filter element must have sufficient area to pass the necessary fluid volume both at the beginning of use when the filter is clean and unobstructed and also after a period of use as the filter becomes partially clogged, the ratio of cardiotomy filter area versus volume for the reservoir taught by the '139 patent is or can be unfavorable. In other words, because of the common diameters of the filters and defoamers in these conventional reservoir designs, the area available to the cardiotomy filter is simply a function of that part of the height of the device which is devoted to treatment of cardiotomy fluid. Thus, the device either has to be made very tall, or most of the height of the device must be allocated to the cardiotomy fluid flow path. As a result, the defoamer area for the venous blood flow path may be undesirably small thereby effecting insufficient bubble removal.
A more recent reservoir design has been proposed in U.S. Pat. No. 5,580,349 to address some of the aforesaid drawbacks. The reservoir design in this patent utilizes a screen, such as a polyester mesh sheet, disposed between two flexible sheets so as to form an input chamber, output chamber and common vent chamber. During use, blood pumped into the input chamber moves upward through the chamber, reaches an apex, and then falls toward the bottom of the reservoir. As the blood moves upward, gas bubbles, including micro-emboli, entrained in the blood are also directed upward. The buoyancy of the bubbles causes the bubbles to escape through the vent chamber as the blood drops to the bottom of the reservoir. In addition, the porous screen which separates the two chambers further enhances bubble separation from the blood as the blood flows along the screen. Although this design provides a means for removing bubbles from cardiotomy fluid and venous blood in a reservoir, it has some inherent shortcomings.
For example, as the fluid and blood fall toward the bottom of the output chamber, at a fluid velocity of approximately 0.9 m/s, backsplash and eddy formation within the reservoir may occur when the fluid and blood hit the bottom of the chamber. As a result, new bubbles may be generated in the output chamber of the reservoir. In addition, the porous surface of the mesh also agitates the downward flowing blood, further contributing to the formation of turbulence and, consequently, bubbles in the fluid and blood. Therefore, there is a potential risk that air or gas bubbles entrained in the fluid and blood may be infused into the patient.
In view of the foregoing, it is apparent that a variety of cardiotomy fluid and venous blood reservoir designs have been developed in an attempt to effectively and efficiently filter and defoam cardiotomy fluid and venous blood prior to being reinfused into the patient. It is also apparent, however, that these reservoir designs still require improvement, particularly as it relates to the reduction and/or elimination of gas and air bubbles in the fluid and blood. In particular, it is apparent that there is a continuing need to provide a blood reservoir with a more effective design that reduces or eliminates turbulence and/or eddy formation and minimizes resultant bubble formation as blood flows through the reservoir. There is also a need to provide a method of treating both cardiotomy fluid and venous blood to safely and effectively remove air or gas bubbles entrained in the blood prior to reinfusing, the blood into a patient.